Is the money a phone repairman receives for repairing phones under warranty by the telecommunications company, with the phone's value refunded to the owner if it cannot be repaired, considered usury?

Being a guarantor for every phone the company sells, and you get 100 for repairing each phone, but if you are unable to repair it, you return its value—this is a clear case of gharar (excessive uncertainty). This is because you could either be at a loss or at a gain, and it is prohibited due to the Sharia's forbiddance of it. Therefore, you must avoid this method.